Different frictional forces and thus frictional losses occur in the bearings according to rotational speed and axial load on the bearings with regard to rotatably mounted bodies. At very high rotational speeds, in particular, which occur in charging devices, for example, in turbochargers, such frictional losses are highly relevant. However, in addition to the forces which arise in the region of viscous friction in the so-called Stribeck curves, the losses, in particular, in the region of boundary friction and mixed friction downstream of the Stribeck, are important. The quality of the mounting depends, among other things, on the behavior of the mounting in the mixed friction region of the Stribeck curve. An exact measurement of the forces arising in this region is therefore desirable, as this would enable a better analysis and evaluation of the signs of wear of the bearing. For an analysis of this type, it is necessary that spatial positions (orbital path, axial movement of the rotating body) and also the frictional torque or the frictional power may be exactly measured or determined under actual conductions and if possible without disturbance variables.
In the industry, diverse test benches are known for measuring frictional power or friction torque (also known as friction power test benches). With regard to these known test benches, different approaches are followed to determine the frictional power and thus the power loss in charging devices.
For example, there are test benches in which the principle of direct frictional measurement is used. This means that the power supplied by a drive is compared with the power arriving at the test object. With regard to known friction power test benches, the operation of the shaft is carried out via a drive in the form of an electromotor, which is coupled to the shaft via coupling elements. Test benches of this type, however may generally realize rotational speeds of only 80,000 rpm, in many embodiments up to 125,000 rpm. This means that not all actual conditions, e.g. of a turbocharger, may be mapped, and the results must be extrapolated instead in order to analyze the rotational speeds possible in actual use of up to 300,000 rpm. In addition, in these test benches, the deviation of the power adjusted at the drive is compared with the power output at the measuring device, and thus the power loss is determined purely mathematically. Consequently, there is no direct measurement of the friction torque. The air friction generated and the mass inertia of the turbocharger rotor should thereby be considered. In order to avoid distortions in the results, these variables must be determined at the beginning of the test and calculated along with it. A further disadvantage is that only one specific, pre-selected axial load may be used for the measurement. In addition, the coupling at the electromotor influences the displacement path (orbital path) of the shaft.
A known alternative for determining the frictional power includes test benches, on which the measurement is carried out in the form of deceleration tests. An axial and a radial force are thereby applied, e.g. using nozzles. After the test object has been accelerated to a specific speed via a drive charger, it is decoupled from the same, and the deceleration test is subsequently executed. This is carried out, for example, using a torque sensor on the test object. An extrapolation for higher speeds is also necessary for measurements of this type. In addition, the measurements are only possible with specifically modified core assemblies or with core assemblies which actually allow a modification. It is additionally disadvantageous that the orbital path is distorted during the acceleration due to the coupling. Similarly, the generation of radial forces using nozzles may not be directly translated to actual operation.
The disadvantages of such known test benches, which have negative influences on the measured results, are, in summary, in particular, the altered mass inertia of the test object, e.g. due to necessary modifications; the rigid connection between drive and supercharger; and the fact that the results for high rotational speeds are not measured but may only be determined through interpolation.
One goal of the present invention is consequently the provision of a test bench which enables a high resolution measurement of the forces occurring in the bearing, in particular, in the region of the mixed friction of the Stribeck curves, under the most actual and influence-free conditions. Additional advantages of the invention will be described in the following.